Rolling mills for hot-rolling metal are well known in the art. Examples are shown in U.S. Pat. Nos. 3,257,835, 3,317,994, 3,296,682, 3,517,537, 3,672,199, 3,766,763, 3,881,336, 3,881,337, 4,087,898, 4,106,319, 4,159,633 and 4,193,823. Such rolling mills normally roll metal stock such as bar or rod between pairs of smooth finished work rolls in tandem roll stands. The work rolls are usually made of a tool steel selected from the following AISI classes: the chromium hot work tool steels H11 through H16, the tungsten hot work tool steels H20 through H26 and the molybdenum hot work tool steels H41 through H43. It is preferred that the work rool material be selected from chromium hot work tool steels H11 to H16 because of their ability to resist heat softening during continuous exposure to high temperatures. While the industry normally employs smooth finished work rolls, a textured work roll of this type tool steel is disclosed in U.S. Pat. No. 4,193,823.
Many types of lubricants have been developed for lubricating the surfaces of the work rolls in a hot rolling mill to reduce roll wear due to abrasion. The lubricants are combined with water to form a coolant-lubricant system which cools the hot metal rod while lubricating the surfaces of the work rolls. These lubricants are conveniently divided into two major groups: (1) those which form heterogeneous aqueous mixtures, i.e. more than one phase; and (2) those which form homogeneous aqueous solutions or apparent solutions, i.e. one phase.
Lubricants of group (1) are normally thought to have relatively low lubricity and relatively low wetting ability. They also are nonpolar and thus must be synthetically suspended in water (which is polar) by emulsifying agents. Group (1) lubricants are therefore normally referred to in the art as oil-in-water emulsion lubricants.
Oil-in-water emulsion lubricants form a suspension of lubricant material in water, are milky white in color, and are opaque. The lubricant base is normally refined mineral oil to which are added an emulsifier agent and detergent, so that the lubricant will form tiny, suspended droplets of various diameters when mixed with or added to water. Typical brand names of examples of this type lubricant are Dromus B, Prosol 68, and Soluble Oil D. Since emulsion lubricants are least expensive, conventional oil-in-water emulsion systems have long been attractive from a cost standpoint and generally preferred in high volume, high make-up systems. When used for cooling lubrication in mild to medium duty applications, oil-in-water lubricants are usually found to be an acceptable choice. In extreme pressure, high temperature service such as hot rolling, satisfactory lubrication and extended roll life are in jeopardy because the typical oil-in-water lubricant is subject to failure. As previously mentioned, this type lubricant mixture is comprised of minute droplets of non-uniform size and held in water suspension by the action of emulsifier agents. The ability to lubricate metal surfaces by the usual means thereby becomes dependent on sufficient numbers of these lubricant droplets transferring from the water carrier medium and attaching themselves to all parts to be lubricated or, more specifically, the smooth finished roll work surfaces. Furthermore, it is established that this ability to "plate out" or "wet" smooth finished metal surfaces is not shared by all lubricant droplets but is characteristic of only a few whose physical size fall within a relatively narrow range of diameters. In general, of the total lubricant content expressed as per cent volume of the working emulsion, only a very small amount is actually beneficial in reducing roll wear. High temperature, dissolved metal ions, hard water ions, gear box lube contamination, mechanical shear forces, and improper pH control are all forces which act to segregate the size of droplets to levels outside the range which is known to be useful. Considering the above description of lubricant dispersion in water, the mechanics of lubricant transfer to metal surfaces, and the comparatively low lubricant potential available even under conditions thought to be ideal in the prior art; oil-in-water emulsion systems have been considered by the industry to be inadequate in providing lubrication and roll life improvement. A better alternative was thought to be found in the more expensive water miscible rolling lubricant of group (2).
Group (2) lubricants are either soluble in water or naturally disperse in water into colloidal particles or droplets generally from about 10 angstroms to about 20,000 angstroms in size. Since lubricants of group (2) are actually or apparently miscible in water, they are referred to in the art as miscible or true solution lubricants. These lubricants are polar, have relatively high lubricity and have relatively high wetting ability.
Water miscible lubricants form clear or slightly turbid solutions or mixtures with water. The lubricant base is normally composed of long chained organic compounds such as fatty acids and may also contain various surface active agents such as amine compounds. These materials will disperse themselves uniformly in water as molecular "bits" of patent lubricant compound. Typical brand examples of this type lubricant are Quakerol, and Lube-Well HR. Comprised mainly of synthetic organic ester materials or long chained fatty acids, these polar lubricants have the inherent chemical ability of dividing themselves into molecular "bits" which are normally strongly attracted to metal surfaces which are also polar. Because the water solutions of these lubricants are not dependent on emulsifier agents for controlling various physical and chemical properties, they are generally able to carry out their function of lubrication unaffected by most of the physical extremes of hot rolling. The ability of these polar lubricants to become adsorbed onto the surface of metals is discussed by Douglas Godfrey in Chapter 2 of the Standard Handbook of Lubrication Engineering and by Stanislav N. Postnikov in Chapter 3 of Electrophysical And Electrochemical Phenomena In Friction, Cutting, And Lubrication. It is believed that smooth finished metal surfaces have considerable free energy and polar lubricant molecules are attracted thereto and align generally perpendicular to the metal surface closely together forming a film characterized by high boundry lubricity. Being surface active in nature, it was generally assumed by the industry that boundary film lubrication is dominant and that the additional benefit of roll surface passivation against high temperature oxidation was possible. When used in a hot rolling mill coolant and lubricant system, however, the polar lubricants are mixed with water which is also polar and thus molecules of polar lubricant must compete with molecules of water for space at the surface of the metal work roll, which detracts from boundry lubricating effectiveness. Roll life improvements over conventional oil-in-water systems were realized by the industry with the use of miscible lubricant systems which helped justify the increase in lubrication cost. However, polar lubricants also have limited usefulness in high temperature applications because the polarity induced boundry film is destroyed by extreme heat.
The industry trend has been toward the use of rolling lubricants that form miscible solutions or mixtures in water which are thought to be normally better able to perform the vital role of lubrication because the industry has assumed that they are less subject to influences which inhibit lubrication of metal surfaces than oil-in-water emulsions. The present invention provides means for increasing the lubricating efficiency of oil-in-water emulsion systems to a level exceeding that of conventional miscible solution systems.